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Investigators have been working to identify offspring and herd mates born within a year of the infected cow’s birth. The infected cow was a 12-year-old Brahma cross beef cow.

Twenty-nine adult cows were removed from the herd on Wednesday, the department said. They were taken to a collection site and euthanized, and tissue samples were removed for testing, the department said.

The samples will undergo screening referred to as a “rapid test,” and if results indicate the presence of mad cow disease, two additional tests will be done.

“As we have further information available, we will continue to update the public as to what those results indicate,” said department spokesman Ed Loyd.

The department released the information Friday as part of its ongoing investigation into the infected animal’s history. A brief statement said investigators are still doing an inventory of the herd and sorting “cattle of interest,” which are offspring and herd mates born around the same time.

U.S. changes testing policyThe cow spent its entire life on the same farm, which the government has not identified. It was sold through a livestock market on Nov. 11 but was dead on arrival four days later at a slaughterhouse, according to the department. Because it was dead, it was taken to a pet food plant in Waco, Texas, which did not use it for food but removed brain tissue for testing, the department said.

Rapid tests indicated the presence of mad cow disease in the animal, but results from a different test were negative, and the department announced the animal was free of the brain-wasting disease. Last month, the agency’s internal watchdog ordered a third type of test that came back positive, and a laboratory in England confirmed the results on June 24.

The U.S. changed its testing policy as a result of the new case. Now, both additional tests — immunohistochemistry and Western blot — will be done if rapid tests indicate a sample is suspect.

The Texas animal is the first native case of mad cow disease. The other U.S. case, confirmed in December 2003, was in a dairy cow in Washington state that had been imported from Canada, where investigators believe it ate contaminated feed.

Also under investigation is the feed history of the Texas herd, because the only way mad cow disease is known to spread among cattle is through the feeding of infected brain and nerve tissue. Once commonly used as a protein ingredient, ground-up cattle remains were banned from use in cattle feed in 1997.

Cattle remains are still added to food for chicken, pigs and pets; experts believe mad cow disease won’t spread to those animals.

Bovine spongiform encephalopathy, or BSE, is the medical name for mad cow disease. In humans, eating infected tissue from cows has been linked to variant Creutzfeldt-Jakob disease, a rare but fatal degenerative disease blamed for the deaths of 150 people in Britain, where there was an outbreak in the 1980s and 1990s.